Magnetic resonance imaging (MRI) acquires data in the spatial frequency domain (k-space), which includes multiple lines which may be termed phase encodes. The k-space data set is converted to an image using the fast Fourier transform (FFT). In many applications, the acquisition of k-space is distributed over several applications of the pulse sequence (called ‘segments’). Alternatively, the acquisition of k-space can be acquired all at once, called ‘single-shot’ imaging. Any periodic distortions in k-space may result in artifacts in the final image, where an artifact is a feature that appears in the final image, but is not actually present in the target object.
In MRI, image artifacts often hamper clinical image evaluation. One type of artifact (ghosting) stems from fluids, tissue or other matter with a long longitudinal recovery time, T1, referred to as “long T1 species”. Such long T1 species include pericardial and pleural effusion, cerebrospinal fluid (CSF) (in the brain and spinal canal), and saline in breast implants. Effusions can occur in different parts of the human body, for example surrounding the heart and within the pericardium (pericardial effusions) or in the lungs (pleural effusions).
The ghosting artifact from long T1 species occurs in MR images obtained using an inversion recovery pulse sequence with a segmented acquisition (Patent application US20100219829 “Long T1 Artifact Suppression Techniques for Magnetic Resonance” (Sep. 2, 2010) by Wolfgang G Rehwald, Enn-Ling Chen, Raymond J. Kim). This sequence repeatedly plays out the same scheme of inversion recovery (IR) pulses and readout (RO) events, but acquires a different set of phase encoding lines during each RO. FIG. 1 shows a timing diagram of such a known segmented IR sequence (gated using the ECG in this example) and shows the arrangement of acquired MR data in a raw data space. For long T1 species, if there is not adequate time between subsequent inversion pulses, a periodic distortion of the raw data occurs, which, when Fourier transformed, causes the long T1 species to appear shifted by an offset amount and superimposed on the final image.
These ghosting artifacts can be superimposed on an image as a “ghost” at multiple erroneous locations thereby obscuring patient morphology and true location of a long T1 species. FIG. 2 shows an example of such ghosting artifacts in a patient. In the example, saline in breast implants (arrows 201) has a very long T1 and causes multiple ghosts across the field of view indicated by the arrows 207.
A second type of artifact occurs in single shot imaging which use inversion pulses. If a stack of single shot images are acquired without adequate recovery time between image acquisitions, the image intensity of long T1 species oscillates between images. Since image intensity is usually interpreted as coming from differences in T1 (where brighter contrast is shorter T1 and darker contrast is a longer T1) this can complicate image interpretation.
The source of the artifacts comes from the oscillations in the magnetization of the long T1 species. FIG. 3 shows the magnetization of a short 303, medium 307, and long T1 species 309. The imaging period in this example equals twice the RR interval 311 where each RR interval is the duration of one heart beat. Each “imaging RR” 311 is followed by a “waiting RR” 313 to let the magnetization recover. The short 303 and medium 307 T1 magnetization passes through respective same recovery curves during each RR imaging period and are in “steady state”, but the magnetization of the long 309 T1 signal does not. The long 309 T1 magnetization signal has a different amplitude during each of the RR interval readouts as shown by curve 309 and the long T1 species is not at steady state. The oscillations lead to phase errors in the raw data that result in described ghosting along the phase encoding direction in the image.
The long T1 ghosting artifacts can prevent a clinical diagnosis from being made based on the acquired MR images. For example, a ghost from pleural effusion may be superimposed on a structure of interest such as a long axis view of the heart so that a diagnosis is not possible. Even more problematic, the ghosting may lead to an incorrect diagnosis in for example delayed enhancement (myocardial viability) images. Smaller bright ghosts from the spinal fluid superimposed onto the myocardium may be misinterpreted as infarcts. This may lead to a false positive diagnosis and possibly inappropriate patient treatment.
Known MR systems which use an IR pulse to create image contrast, address the artifact problem by including dummy heartbeats (DHBs) that are played at the beginning of a scan. A DHB sequence is a pulse sequence (in this case, an inversion pulse and RF and gradient pulses that are usually played to acquire data) played in the leading period, but no data is recorded (the recoding event is turned off, giving a dummy readout, DR). Whereas this improves image quality, it does not completely remove the long T1 ghosting. The known “dummy heartbeats” methods weaken the intensity of the ghosting artifact but do not fully remove it since a single dummy period is not enough to drive the long T1 fluid into steady state. In order to fully remove a long-T1 ghosting artifact using only dummy heartbeats one would need at least four dummy heartbeats leading to 4×2=8 additional hearbeats. This exceeds a patient's breath hold capability. Therefore, the known “dummy heartbeats” methods fail to fully prevent the long T1 ghosting artifact.
Another known system places a saturation slab over the region containing the long T1 fluid. This depends on the skill of the scanner operator and can only be done in special cases where the long T1 fluid is not part of the imaged structure. This is not a solution for pericardial effusion as it is a substantial part of the imaged structure (the heart). Even when this method is possible, such as in pleural effusion, it requires scanner operator skill and time to position the saturation slab and adjust its thickness. Additionally, the time delay between the saturation pulse and the center of k-space allows for some magnetization to recover, and artifacts may still occur. In the case where there is more than one region with long T1 fluids, multiple saturation slabs need to be manually placed further complicating scanner operation.
Another known system uses a pre-suppression module that, together with dummy heart beats, works better than dummy heartbeats alone. This method requires a precisely set time delay after the inversion pulse, which is difficult if not impossible to calculate, as parameters needed for the calculation are not exactly known or can vary during the scan. Additionally, the time delay is only optimized for one long T1 species, and if other long T1 species are present this method will not suppress them.
A further known method is a phase sensitive inversion recovery (PSIR). This method uses image processing to remove ghosting artifacts, and does not prevent them from being generated in the first place. Additionally, this method only works when the long T1 species is located in close proximity to one of the RF coils used to acquire the image. A system according to invention principles addresses these and related problems.